An ignition coil is known from practice and is used, in particular, for triggering a spark plug of a motor vehicle's internal combustion engine operating by the spark ignition principle. The ignition coil forms an energy storage device and transformer via which electric power of a relatively low supply voltage, usually provided via a DC on-board electrical system of the particular motor vehicle, is converted into magnetic power, which is in turn converted into a high-voltage pulse at a certain point in time, when an ignition pulse is to be supplied to the spark plug used for igniting a fuel mixture in the combustion chamber of the engine.
The known ignition coil includes a housing in which a magnetically active core made of ferromagnetic material, for example, iron, is situated. The core is surrounded by a first, primary, winding connected to a supply voltage, and a second, secondary, winding connected to a high-voltage terminal of the ignition coil. Both windings are usually made of copper wire.
To convert the voltage delivered from the DC on-board electrical system of the motor vehicle into a high voltage, a current flows through the primary winding, which produces a closed magnetic field surrounding this winding and having a defined polarity. To deliver the stored electric energy in the form of high-voltage pulses, the electric current is turned off, so that the built-up magnetic field is forced to change polarity. This results in a high electric voltage in the secondary winding, which is near the primary winding and has a much higher number of turns than the primary winding. The previously formed magnetic field collapses due to the conversion of the now electric power at the spark plug. The spark plug discharges. Depending on the design of the secondary winding, the high voltage, a spark current, and a spark duration during ignition of the fuel mixture supplied to the combustion chamber of the engine may be adapted to the particular requirements.
To ensure the function of the ignition coil, the high voltage generated in it must be insulated from other electrically conductive parts. The secondary, i.e., high-voltage winding, is usually electrically insulated from other electrically conductive parts by electrically insulating materials and/or by air gaps. If such an insulation is insufficient, a secondary electric contact or an electrical breakdown from the high-voltage winding to another electrically conductive component of the ignition coil may occur. As a result, only a reduced high voltage, which is usually no longer sufficient for performing a reliable ignition at the spark plug, is available at the high-voltage terminal of the ignition coil.
In regard to an electrical breakdown, critical components of the ignition coil may include the windings and in particular components of the magnetically active core. The core is usually grounded, so that a large electric potential difference exists between the secondary winding, which is at high voltage, and the magnetically active core.
The risk of an electrical breakdown is determined not only by the potential difference, but also by the intensity of the electric field applied between the high-voltage winding and the particular electrically conductive component. The electric field intensity is highly dependent on the geometric conditions present. In particular, for physical reasons, corners or pointed surface contours of the particular component result in local increases in the field intensity, which in turn favor an electrical breakdown. Corners, points, or edges on electrically conductive parts situated in the range of influence of the high voltage thus represent a potential electrical breakdown risk in an ignition coil.
The magnetically active core is usually made of stamped individual metal sheets packed to form a stack of a certain height. Therefore, the stack has numerous corner-like or pointed uneven features, in particular at its sides, which may result in strong excessive field intensities and therefore in a breakdown during the operation of the ignition coil. To prevent the occurrence of electrical breakdowns, large insulation distances have been previously required or, when possible, good insulation materials have been used.
An object of the present invention is to provide an ignition coil in which, despite the existence of a rough surface on an electrically conductive component, the risk of an electrical breakdown is small, and which may be manufactured with compact dimensions.